Methylation of DNA cytosine residues results in epigenetic transcriptional repression, and aberrant methylation plays a role in human cancers. During development, DNA methylation is a mechanism whereby cell-type specific gene expression patterns are set during terminal differentiation. However, the specific role of DNA methylation in terminal differentiation and organogenesis has so far been studied in very few cell types. This is due in part to the early embryonic lethality of knockout mice lacking genes required for DNA methylation. Unlike mouse models, zebrafish with mutations in two key epigenetic regulators, DNA Methyltransferase 1 (dnmt1) and Ubiquitin-like, Containing PHD and RING Finger Domains 1 (uhrf1), survive to late embryonic stages, at which time many complex organs (including the eye) have formed. I have taken advantage of these mutant zebrafish lines to study the role of DNA methylation in development of the vertebrate lens. Consisting of only two cell types: proliferative epithelial cells and terminally differentiated lens fibers, the lens is an ideal tissue in which to study gene regulation. Despite this, little is currently known about the role of DNA methylation in lens development. My preliminary data show that loss of Dnmt1 and Uhrf1 function leads to cataracts and morphologically abnormal lenses that contain disorganized and apoptotic lens fibers. Methylation of genomic DNA in dnmt1 and uhrf1 mutants is reduced to 25% of wild type levels, supporting a model in which DNA methylation is required for normal lens development. The goal of the proposed research is to determine how DNA methylation functions to silence genes during lens fiber terminal differentiation. This will be determined with the following specific aims: Specific Aim 1: To determine whether lens epithelial cell genes, which are downregulated during lens fiber terminal differentiation, are silenced by DNA methylation. Specific Aim 2: To determine whether the lens defects in dnmt1 and uhrf1 mutant zebrafish are caused by deficient histone H3K9 tri-methylation. The experiments proposed are novel in that they will begin to elucidate the role and mechanism of DNA methylation-mediated gene silencing in the process of lens formation. This topic has relevance as a mechanism for understanding cataract formation, and it will increase our understanding of how deregulated DNA methylation can contribute to cancer.